I don't know much about Charles Williams I have to admit but I do know that there's an old work that examined the allegorical nature of his works entitled
Some aspects of allegory in the novels of Charles Williams The reason I know this title so well is because a friend of mine who knows a lot more about this author than I ever will has been trying to obtain a copy of it, so perhaps there's something in what Mr Worthington is alluding to.
I'm sure he'll arrive soon enough and provide an explanation in his trademark articulate style...
Then again it could be a matter of interpretation like you said?